{-# LANGUAGE PatternGuards #-}

-- | Advance the world to the next time step.
module Advance where
import World
import Contact
import Collide
import Actor
import Config

import Graphics.Gloss.Data.Vector
import Graphics.Gloss.Geometry.Angle
import Graphics.Gloss.Interface.Pure.Simulate
import qualified Graphics.Gloss.Data.Point.Arithmetic as Pt

import qualified Data.Map       as Map
import qualified Data.Set       as Set
import Data.Map                 (Map)


-- Advance ----------------------------------------------------------------------------------------

-- | Advance all the actors in this world by a certain time.
advanceWorld
        :: ViewPort             -- ^ current viewport
        -> Time                 -- ^ time to advance them for.
        -> World                -- ^ the world to advance.
        -> World                -- ^ the new world.

advanceWorld viewport time (World actors tree)
 = let
        rot             = viewPortRotate viewport
        force           = rotateV (degToRad rot) (0, negate gravityCoeff)

        -- move all the actors
        actors_moved    = Map.map (moveActor_free time force) actors

        -- find contacts in the world
        (contacts, tree')
                = findContacts (World actors_moved tree)

        -- apply contacts to each pair of actors
        actors_bounced
                = Set.fold
                        (applyContact time force)
                        actors_moved
                        contacts

   in   World actors_bounced tree'


-- Move two actors which are known to be in contact.
applyContact
        :: Time                 -- ^ time step
        -> Force                -- ^ ambient force on the actors
        -> (Index, Index)       -- ^ indicies of the the two actors in contact
        -> Map Index Actor      -- ^ the old world
        -> Map Index Actor      -- ^ the new world

applyContact time force (ix1, ix2) actors
 = let  -- use the indicies to lookup the data for each actor from the map
        Just a1 = Map.lookup ix1 actors
        Just a2 = Map.lookup ix2 actors

        resultActors
                -- handle a collision between bead and a wall
                | Bead _ _ r1 p1 v1     <- a1
                , Wall{}                <- a2
                = let   a1'             = collideBeadWall a1 a2
                  in    Map.insert ix1 a1' actors

                -- handle a collision between two beads
                | Bead ix1 m1 r1 p1 v1  <- a1
                , Bead ix2 m2 r2 p2 v2  <- a2
                = let
                        (a1', a2')
                                -- if one of the beads is stuck then do a safer, static collision.
                                -- with this method the beads don't transfer energy into each other
                                -- so there is less of a chance of lots of beads being crushed
                                -- together if there are many in the same place.
                                | m1 >= beadStuckCount || m2 >= beadStuckCount
                                = let   a1'     = collideBeadBead_static a1 a2
                                        a2'     = collideBeadBead_static a2 a1
                                  in    (a1', a2')

                                -- otherwise do the real elastic collision
                                --      this is much more realistic.
                                | otherwise
                                = collideBeadBead_elastic a1 a2

                        -- write the new data for the actors back into the map
                  in    Map.insert ix1 a1'
                   $    Map.insert ix2 a2' actors

   in   resultActors


-- | Move a bead which isn't in contact with anything else.
moveActor_free
        :: Time                 -- ^ time to move it for
        -> Force                -- ^ ambient force on the actor during this time
        -> Actor                -- ^ the bead to move
        -> Actor                -- ^ the new bead

moveActor_free time force actor
        -- move a bead
        | Bead ix stuck radius pos vel  <- actor
        = let   -- assume all beads have the same mass.
                beadMass        = 1

                -- calculate the new position and velocity of the bead.
                pos'            = (pos Pt.+ time  `mulSV` vel)
                vel'            = (vel Pt.+ (time / beadMass) `mulSV` force)

                -- if the bead is travelling slowly then set it as being stuck.
                stuck'
                 | magV vel' < 20
                 = min beadStuckCount (stuck + 1)

                 | otherwise
                 = max 0 (stuck - 2)

          in  Bead ix stuck' radius pos' vel'

        -- walls don't move
        | Wall{}                        <- actor
        = actor